As the semiconductor industry moves towards higher circuit densities and larger memory sizes (such as 64 megabit dynamic random access memories "DRAMs" and 256 megabit DRAMs) more stringent requirements are being placed on low frequency noise suppression (also known as power line de-coupling) and noise filtering systems on printed wiring boards (PWBs), or printed circuit boards. De-coupling, or bypass, capacitors are necessary to provide a temporary supply of charge to the active integrated circuits as the output of power supplies connected to the integrated circuits varies. In this way, circuit operation is not compromised due to a temporary drop in voltage supply. De-coupling capacitors are typically in the 0.5 to 1 microfarad range. They are located between the power supplies and the ground. The trace on the board can be at the power supply voltage and the pin linked to circuit ground or the reverse. To enhance de-coupling, these bypass capacitors need to be as close to the active devices as possible in order for the inductance between the capacitors and the devices to be as small as possible. As circuits get more complex the number of power supplies increase and so the need for capacitors.
Resistors are needed to damp the "ringing" which is mostly due to the impedance mismatch and noise in the transmission line. A series resistor damps and isolates the noise on the signal line. Usually it is used close to the "t" junction where more noise and impedance mismatch is found. Also, a normal TTL driver has an impedance of around 20 ohms and the signal line on the PCB has a 50-70 ohms impedance. Connecting them directly would result in a major mismatch which will affect signal integrity on the line. A series resistor will improve the match and as a result reduce the ringing. Also for TTL or LVTTL using a series resistor does not affect the power on output voltage level of the driver significantly while it helps signal integrity. Usually, these resistors go in the input/output lines with values typically 10-15 ohms.
The use of inductances in surface mount is rare and mostly reserved to filter circuits in analog circuits. But when needed they can also go under the lead.
As integrated circuits get smaller and smaller and as the lead pitch of active components get smaller and smaller there is less and less space on the board for passive components such as resistors, capacitors and inductors. In particular, de-coupling capacitors are a problem since each memory integrated circuit needs at least one. Also, resistors used in the input and outputs of memory devices are becoming more of a problem since memories are offered in a variety of configurations such as X1, X4, X8, X16, X32 and X64. Such configurations are typically packaged in SIMM modules with each DRAM having about 2200 leads.
Prior art FIG. 1 illustrates a present passive component which is generally representative of a resistor, capacitor or inductor. The passive component 10 is generally in the shape of a parallepiped having a body 12 with terminations 14a and 14b on each end of the body. The terminations 14a and 14b are metallic for the purpose of making electrical connection and usually are located at the short section ends of component 10 in either an annular or terminal shape. The capacitor of prior art FIG. 1 would typically have a ceramic body 12. Prior art FIG. 2 depicts the passive component 10 mounted to a printed wiring board 16 and an integrated circuit device 18 mounted to the printed wiring board 16. The passive component 10 is mounted to board 16 with its long dimension parallel to board 18; ie, it is mounted horizontally. Terminations 14a and 14b are attached to copper covered by solder lands 16a and 16b on board 16. Lead finger 18a of integrated circuit device 18 is attached to land 16b. The lead finger is illustrated in the typical shape. The lead fingers and passive component terminations would typically be attached to the lands of printed wiring board 16 in a solder reflow operation. While prior art FIGS. 1 and 2 depict passive component 10 as a parallelpiped, other passive components have a cylindrical shape that are mounted with their long axis parallel to the printed board.
The shape, size and orientation of the present passive electrical components forces the user to mount them adjacent to the active devices which waste a great deal of space on the assembly. The option of mounting the passive components under the bodies of the active devices does not exist anymore since the active devices have thinned down and sit within mils of the PWB or sit on the PWB. The option of imbedding the passive components in the PWB does not exist anymore because of layout reasons. Other options include incorporating the passive components into the integrated circuit package as disclosed in U.S. Pat. No. 5,115,298 issued on May 19, 1992 to Wah K. Loh, assigned to Texas Instruments Incorporated and as disclosed in U.S. Pat. No. 4,598,307 issued on Jul. 1, 1986 to Wakabayashi et al., assigned to Fujitsu.
What is needed is a solution that saves printed wiring board space while placing the passive component as close as possible to the active device.
It is accordingly an object of this invention to provide a passive component that can be mounted to a printed wiring board in close proximity to the active electrical device and while saving space over prior art externally mounted passive devices.
Other objects and advantages of the invention may be apparent to those of ordinary skill in the art, having the benefit of the following specification and drawings herein.